Heat butter in a frypan and sizzle bacon until starting to crisp. Add half the greens and stir over heat until wilted, about 30 seconds. Lightly whisk eggs, season with salt and pepper and mix in feta.
Pour into pan, tilting to spread egg evenly. Loosen a couple of times with a knife or spatula at the start of cooking to allow raw egg to go to the base of the pan.
As mixture sets, sprinkle with mozzarella and top with rest of greens. Cook until set, another minute, then fold in half. Squeeze over lemon juice and serve. Serves 1 as a main meal.